Combustion engines whose combustion gases are routed through a catalytic converter often come equipped with what is known as a secondary-air system. The secondary-air system is provided for more rapid heating of the catalytic converter to a service temperature in a cold-start phase of the combustion engine, by supplying additional air into an exhaust-gas section.
The diagnosis of the tightness of the secondary-air system is usually performed with the aid of the Lambda probe in the exhaust-gas section of a combustion engine. In the process, the secondary-air mass in the exhaust-gas section is calculated with the aid of a generally applicable algorithm. However, this diagnosis possibility of the secondary-air system has the disadvantage that, starting at the instant when the Lambda probe is operative, it is dependent upon the thorough mixing of the secondary air with the combustion gases as well as on the mixture-side pilot control errors during the cold-start phase. New legal provisions mandate that in the future the secondary-air diagnosis can be performed only during the cold-start phase. This is critical in particular in the case of systems that supply the secondary air into the exhaust-gas section only for a brief period of time during the cold-start phase, and also in the case of systems that are equipped with an exhaust-gas turbocharger, since in these cases the Lambda probe reaches its operativeness too late to perform the diagnosis while still in the cold-start phase.
Another possibility for diagnosing the secondary-air system consists of analyzing the pressure in the secondary-air system. To do so, a pressure sensor is installed between the secondary-air pump and the secondary-air valve, and the measured secondary-air pressure is analyzed in order to detect a leak in the secondary-air system by a threshold-value comparison.
The conventional algorithm for detecting an error in the secondary-air system is implemented in that, once the secondary-air pump is turned on, the instantaneously prevailing pressure is measured, noted and the difference is then determined between the instantaneously applied pressure and the pressure upon activation of the secondary air pump. This method has the advantage of eliminating a possibly existing offset error of the pressure sensor. However, detecting a leak downstream from the secondary-air valve with this diagnosis algorithm is error-prone since the secondary-air valve constitutes a throttle for the secondary-air stream, across which a relatively high pressure drop occurs. That means that the pressure drop that occurs on account of a leak downstream from the secondary-air valve is relatively minor in comparison with the pressure drop in the case of a leak upstream from the secondary-air valve. The leak downstream from the secondary-air valve is therefore able to be diagnosed only with a lot of effort and by using very sensitive pressure sensors in the secondary-air system. Furthermore, a relatively low value would have to be specified for the error-detection threshold, so that an insignificant leak upstream from the secondary air valve does not already have the undesired effect of causing the detection and signaling of an error.
Therefore, it is the object of the present invention to provide a method and a device for diagnosing the secondary-air system, with whose aid a leak in the exhaust-gas section especially downstream from the secondary-air valve is able to be detected and, in particular, significant errors between the secondary-air pump and the secondary-air valve are likewise able to be detected in a reliable manner.